I know this applies, but installing gnome-shell pulls in gdm.
I.e. removing gdm without removing gnone-shell is not possible.
Because gnome-shell (running in a special mode) is nowadays the greeter used
by GDM. That does not mean GDM won't let you log into KDE if you have it
installed.
As for GDM requiring gnome-shell, I don't think it should come across as
surprising because GDM is GNOME's display manager. If you dislike GNOME so
much then get yourself a different display manager.
Cheers,
Debarshi
--
If computers are going to revolutionize education, then steam engines and cars
and electricity would have done it too. -- Arjun Shankar